1. Field of the Invention
The invention relates in general to output power switching regulators for power sources containing a single maximum power point, and relates in particular to regulators where the primary power source is combined with a secondary power source, such as a battery.
The invention is of special relevance with respect to battery augmented solar array power systems in which control signals from the pulse width modulated power regulator are essentially derived from a monitoring of an output current such as the battery current. The nominal system condition has the solar array supplying power through a regulator to a varying load while also supplying power to charge the battery.
While the subject invention will be described with reference to particularized embodiments and end uses, the invention is not limited to such embodiments and uses. Those having ordinary skill in the art and access to the teachings of this specification will recognize additional implementations and utilizations within the scope of the invention.
2. Background Art
In any system containing multiple power sources, a regulator is used to determine how the power sources will be used to satisfy the load demands for power. If the available power from the primary power source exceeds the demand, the regulator can be used to adjust that power source to operate at a point below maximum capability. If the load exceeds the power available from the primary power source, the regulator can adjust the primary power source to operate at its maximum power point and use the secondary power source to satisfy the excess demands.
An example of such a system is one where the primary source is a solar array and the secondary source is a battery. If the solar array fully satisfies the demand by the loads for power, the regulator maintains the battery in a fully-charged, ready condition. If the array is unable to satisfy the demand loads, the regulator adjusts the array to operate at the maximum power point, and the battery then supplies the remaining load demand. However, it is critical that the regulator be able to quickly determine whether the loads exceed the capabilities and adjust accordingly.
The prior art discloses several techniques for locating the maximum power point. All techniques are based on the mathematical relationships between power P, voltage V and current I: ##EQU1## In particular, the maximum power point can be located by setting ##EQU2##
However, in order to provide real time adjustments of the operating point, this equation must be solved quickly. This is difficult because solving this equation requires at least eight calculations and measurements: (1) a determination of present array voltage V; (2) a determination of present array current I; (3) a measure of the change in voltage, dV, in the face of a given operating point perturbation (dt); (4) a measure of the change in current, dI, corresponding to the operating point perturbation; (5) a calculation of the product V.times.dI; (6) a calculation of the product I.times.dV; (7) a calculation of the resulting sum VdI+IdV; and (8) a comparison of this sum to an equal perturbation on the opposite side of the operating point or the operating point power. Furthermore, if the final sum is not zero, a ninth determination must be made of the sign of the dP sum. This sign indicates the direction that the operating point must be adjusted to reach the maximum power point.
Prior art regulators take at least three general approaches to solving this complex equation: trial and error techniques using (1) analog or (2) digital processing; or (3) an a priori technique based on a known reference. All of these techniques rely heavily on simplifying assumptions.
The first approach solves this equation using a trial and error technique to locate the peak power point. This technique is based on the fact that the output power of the solar array is a continuous function of voltage and current with a single peak power point. The technique uses analog circuits and involves iteratively perturbing the array voltage and current, known as the operating point, monitoring the change in array voltage and current, noting resultant changes in array output power, and steering the operating point to a point where until such changes are found to be essentially equal. That point is the maximum power point.
The major problem with analog processing of solar array output parameters is the sensitivity of voltage and current sensors to noise. These sensors must have a wide dynamic range but the signal of interest may only change minutely. Using the current sensor as an example, a first current sensor would measure on a scale from 0 amperes to 50 amperes; a second would measure on a scale from 0 amperes to 1 ampere with a variable bias of up to 50 amperes. The bias would have to settle down for a fixed time to insure little error.
In addition to the problem of noise sensitivity, this approach is limited by the complexity of regulator input and output voltage and current sensors, as well as the complexity of the calculation of the power function.
In the digital approach, information from sensors is gathered from analog sensors. The equation is solved digitally by some form of computer and control signals are returned to the regulators. Digital systems also have major disadvantages. They require analog-to-digital conversion modules, computation memory modules and reverse digital-to-analog conversion modules and other hardware. In addition to the direct cost of such equipment, the intrinsic weight, volume and power consumption penalties can be especially disadvantageous in such critically limited operational environments as spacecraft.
Another important disadvantage of digital processors is the resultant slow speed at which such systems perform the required operating point movement. The total amount of time required for complete cycles of (1) pre-calculation analog-to-digital conversion of sensed analog parameters, (2) memory access and transfer of computationally-required stored parameters, (3) actual microprocessor calculations, (4) retransfer to memory of calculation results, and (5) post-calculation digital-to-analog conversion of resultant control parameters, can make the microprocessor approach quite slow. The result is that the overall system will be inefficient when power demands are most critical.
Finally, the a priori approach uses information from tests of solar cells or test of arrays similar to the controlled solar array and moves the operating point to the predicted peak power point. Serious errors can result if the conditions of test vary from the application. Since peak power points change with life, heat, light intensity, shadowing and a host of other parameters, this technique has not been widely used.
It is accordingly a general object of the present invention to significantly improve the overall performance of the primary power source in a system containing multiple power sources.
It is a more specific object of the present invention to significantly improve the net system efficiency of the output power regulators in such systems.
It is a further specific object of the present invention to increase the speed of such regulators so as to thereby increase associated overall system responsiveness to rapid fluctuations in operational conditions.
It is a still more specific object of the present invention to reduce the complexity and noise sensitivity of a regulator by sensing the input current of the battery.
It is yet another object of the present invention to reduce the associated cost, weight, volume and power consumption of required regulator components.